The present invention pertains generally to devices and methods for separating and segregating the constituents of a multi-constituent material. More particularly, the present invention pertains to devices for efficiently initiating and maintaining a multi-species plasma in a chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios. The present invention is particularly, but not exclusively, useful as a filter to separate the high mass particles from the low mass particles in a plasma that is initiated and maintained by high frequency wave heating.
There are many reasons why it may be desirable to separate or segregate mixed materials from each other. One such application where it may be desirable to separate mixed materials is in the treatment and disposal of hazardous waste. For example, it is well known that of the entire volume of nuclear waste, only a small amount of the waste consists of radionuclides that cause the waste to be radioactive. Thus, if the radionuclides can somehow be segregated from the non-radioactive ingredients of the nuclear waste, the handling and disposal of the radioactive components can be greatly simplified and the associated costs reduced.
Indeed, many different types of devices, which rely on different physical phenomena, have been proposed to separate mixed materials. For example, settling tanks which rely on gravitational forces to remove suspended particles from a solution and thereby segregate the particles are well known and are commonly used in many applications. As another example, centrifuges which rely on centrifugal forces to separate substances of different densities are also well known and widely used. In addition to these more commonly known methods and devices for separating materials from each other, there are also devices which are specifically designed to handle special materials. A plasma centrifuge is an example of such a device.
As is well known, a plasma centrifuge is a device which generates centrifugal forces to separate charged particles in a plasma from each other. For its operation, a plasma centrifuge necessarily establishes a rotational motion for the plasma about a central axis. A plasma centrifuge also relies on the fact that charged particles (ions) in the plasma will collide with each other during this rotation. The result of these collisions is that the relatively high mass ions in the plasma will tend to collect at the periphery of the centrifuge. On the other hand, these collisions will generally exclude the lower mass ions from the peripheral area of the centrifuge. The consequent separation of high mass ions from the relatively lower mass ions during the operation of a plasma centrifuge, however, may not be as complete as is operationally desired, or required.
Apart from a centrifuge operation, it is well known that the orbital motions of charged particles (ions) in a magnetic field, or in crossed electric and magnetic fields, will differ from each other according to their respective mass to charge ratio. Thus, when the probability of ion collision is significantly reduced, the possibility for improved separation of the particles due to their orbital mechanics is increased. For example, U.S. Pat. No. 6,096,220, which issued on Aug. 1, 2000 to Ohkawa, for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d and which is assigned to the same assignee as the present invention, discloses a device which relies on the different, predictable, orbital motions of charged particles in crossed electric and magnetic fields in a chamber to separate the charged particles from each other. In the filter disclosed in Ohkawa ""220, the magnetic field is oriented axially, the electric field is oriented radially and outwardly from the axis, and both the magnetic field and the electric field are substantially uniform both azimuthally and axially. As further disclosed in Ohkawa ""220, this configuration of fields causes ions having relatively low mass to charge ratios to be confined inside the chamber during their transit of the chamber. On the other hand, ions having relatively high mass to charge ratios are not so confined. Instead, these larger mass ions are collected inside the chamber before completing their transit through the chamber. The demarcation between high mass particles and low mass particles is a cut-off mass Mc which is established by setting the magnitude of the magnetic field strength, B, the positive voltage along the longitudinal axis, Vctr, and the radius of the cylindrical chamber, xe2x80x9caxe2x80x9d. Mc can then be determined with the expression: Mc=ea2(B)2/8Vctr.
Generally, for most plasma related applications, energy must be expended to initiate and maintain the plasma. Considerable effort has been made to minimize the energy required to initiate and maintain the plasma. Heretofore, electron cyclotron heating (ECH) processes, wherein an electromagnetic wave is launched into a plasma chamber to initiate and maintain the plasma, have been developed for plasma deposition applications (see for example, Principles of Plasma Discharges and Materials Processing, by Lieberman, Wiley Interscience, pgs. 412-415).
The general dispersion relation for a wave propagating in plasma can be written:
Tan2xcex8=xe2x88x92K∥(N2xe2x88x92Kr)(N2xe2x88x92K|)/((N2xe2x88x92K∥)(Kxe2x8axa5N2xe2x88x92K|Kr))xe2x80x83xe2x80x83[1]
where xcex8 is the angle of the wave propagation relative to the magnetic field, B, N is the index of refraction (i.e., N=ck/xcfx89) where c is the speed of light, k is the wave vector, ne is the electron density, e is the electron charge, and xcfx89 is the wave frequency); and for frequencies much greater than ion cyclotron and ion plasma frequencies:
Kr=1xe2x88x92xcfx89p2/(xcfx89(xcfx89xe2x88x92xcfx89c))
K|=1xe2x88x92xcfx89p2/(xcfx89(xcfx89+xcfx89c))
K195=1xe2x88x92xcfx89p2/(xcfx892xe2x88x92xcfx89c2))
K∥=1xe2x88x92xcfx89p2/xcfx892 
where wc=eB/mc=1.8xc3x971011 B and wp2=ne2/(xcex50mc)=57 n1/2 are the electron cyclotron and electron plasma frequencies.
For propagation along the magnetic field, xcex8=0, the numerator of Eq, [1] must vanish and for propagation at xcex8=xcfx80/2 the denominator must vanish. These solutions give the principal waves. The right-hand polarized wave rotates in synchronism with the electrons when xcfx89=xcfx89ce leading to resonant energy absorption. Collisional absorption can also be effective and can be estimated by substituting xcfx89=xcfx89+ixcexd. The physics of high frequency wave propagation and absorption lead to two approaches for heating the plasma 5 mass filter with electron cyclotron waves. The first approach utilizes a resonant wave that is launched along the magnetic field with the magnetic field chosen to decrease away from the launcher and the resonant field is located axially at a point where the heating is desired. The second approach utilizes a wave propagating radially in a cavity perpendicular to the magnetic field, (xcex8=xcfx80/2); this requires a high frequency wave above the electron plasma frequency and relies on collisional absorption. For the case of xcex8=0, choosing the wave synchronous with the electrons allows Eq. [1] to be written:
k2/k02=1xe2x88x92xcfx89p2/(xcfx89(xcfx89xe2x88x92xcfx89c))xe2x80x83xe2x80x83[2]
where k0=w/c 
and for perpendicular propagation, xcex8=xcfx80/2, the dispersion relation can be written:
k2-/k02=K1Kr/K195 xcx9c(1xe2x88x92xcfx89p2/xcfx892) for xcfx89 greater than xcfx89c.xe2x80x83xe2x80x83[3]
For the case of a wave launched along the magnetic field from one end of the device, xcex8=0. The dispersion relation shows that for regions where xcfx89 less than xcfx89c, the circularly polarized wave propagates at any plasma density and for regions where xcfx89=xcfx89c, the circularly polarized wave is strongly damped. For regions where xcfx89=xcfx89c, a resonance zone occurs and ionization and heating of gas/plasma occurs. Furthermore, the placement of the resonance zone within the chamber can be controlled by the proper distribution of the magnetic field within the chamber.
For example, consider a chamber having a magnetic field that diverges from a first end of the chamber where the magnetic field is B1 to the middle of the chamber where the magnetic field is B0, and where B1 greater than B0. When a circularly polarized wave having a frequency, xcfx89,
xcfx89=eB0/mxe2x80x83xe2x80x83[4]
is launched from the first end toward the middle of the chamber, the circularly polarized wave propagates to the resonance zone. This is because xcfx89 less than xcfx89c for regions where B greater than B0. Furthermore, the circularly polarized wave is strongly absorbed at the resonance zone where B=B0 and xcfx89=xcfx89c. At the resonance zone, heating and ionization of the plasma occurs because the rotating electric field of the circularly polarized wave matches the gyrating orbits of the plasma electrons. Thus, the electrons receive essentially a static electric field which imparts a large acceleration on the electrons. Collisions between the accelerated electrons and other electrons and ions result in heating.
For an exemplary circularly polarized wave of frequency 2.45 GHz (i.e. a wave in the microwave spectrum), the resonance field is approximately B0=0.085 T. For a plasma density, n, of 1018mxe2x88x923 the plasma frequency is xcfx89p=5.7xc3x971010/s.
For the case of a wave launched perpendicular to the magnetic field, xcex8=xcfx80/2, the frequency has to be chosen high enough to insure wave propagation and the absorption is not resonant, but collisional. Since collisional absorption is generally not strong for conditions of interest, it is important to have the plasma immersed in a high Q cavity in order to get efficient heating. A choice of frequency just above the electron plasma frequency at the desired operating point is a good choice. The electron collision frequency for 1eV electrons is about xcexdxcx9c2.9xc3x9710xe2x88x9211n, or about 2.9xc3x97107 for a density n of 1018 mrxe2x88x923 giving a ratio of xcexd/xcfx89pxcx9c5xc3x9710xe2x88x924. An estimate of the damping length from the imaginary part of the wave vector for xcfx89 greater than xcfx89p greater than xcfx89c gives:
ki=(xcexd/2c)xcfx89p2)/w2)/(1xe2x88x92xcfx89p2/w2)1/2xcx9c(xcexd/2c)(xcfx89p2/xcfx89p2)xcx9c5xc3x9710xe2x88x9220 n(xcfx89p2/w2)mxe2x88x921xe2x80x83xe2x80x83[5]
Hence, the damping length is of order 100 m, so it is desirable to have the cavity Q high enough to allow on the order of 103 transits of the wave to insure adequate damping.
In light of the above, it is an object of the present invention to provide devices and methods suitable for the purposes of efficiently initiating and maintaining a multi-species plasma in a chamber and then separating the ions in the multi-species plasma according to their respective mass to charge ratios. It is another object of the present invention to provide a heating source for a plasma mass filter in which the location within the plasma of the effective heating zone can be adjusted by varying the magnetic field distribution within the filter. It is still another object of the present invention to provide a heating source for a plasma mass filter that does not require high voltage components inside the plasma chamber that would otherwise be subject to breakdown in poor vacuum conditions. Yet another object of the present invention is to provide devices and methods for separating the constituents of a multiconstituent material which are easy to use, relatively simple to implement, and comparatively cost effective.
In overview, the present invention is directed to devices and methods for separating and segregating the constituents of a multi-constituent material. In particular, for the operation of the present invention, a multi-species plasma is first created from the multi-constituent material using high frequency wave heating. Once the multi-species plasma is created, crossed electric and magnetic fields are used to separate ions in the plasma having a relatively low mass to charge ratio from ions in the plasma having a relatively high mass to charge ratio.
In greater detail, the device in accordance with the present invention If includes a chamber having a substantially cylindrical wall that extends between a first end of the chamber and a second end of the chamber. The cylindrical wall is centered on a longitudinal axis. Magnetic coils are selectively arranged on the outside of the chamber wall and are activated to generate a magnetic field inside the chamber that is directed substantially along the longitudinal axis. In a first embodiment, a magnetic field is established in the chamber that diverges from a magnitude B1 at the first end of the chamber to a magnitude B0 at a zone between the first end and the second end of the chamber, with B1 being greater than B0 (B1 greater than B0). From the zone where the magnitude is approximately B0, the magnetic field can converge to the second end where the magnetic field has a magnitude B2, with B2 being greater than B0 (B2 greater than B0), and accordingly (B1 greater than B0 less than B2). In one implementation, the magnetic field has substantially the same magnitude at both the first and second ends of the chamber (B1=B2). With this cooperation of structure, the magnetic field decreases in magnitude from the first end of the chamber to the zone, while also decreasing in magnitude from the second end of the chamber to the zone. Importantly for this embodiment, a zone having a magnetic field strength of magnitude of B0 is created in the chamber between the first and second end of the chamber wall.
Continuing with the first embodiment, one or more launchers are provided at the end(s) of the chamber to launch circularly polarized electromagnetic wave(s) into the chamber in a direction substantially parallel to the longitudinal axis. For the present invention, the circularly polarized electromagnetic wave(s) are created having a frequency xcfx89, where xcfx89=eB0/m and e/m is the electron charge/mass ratio (e/m=1.8xc3x971011 coul/kg). Furthermore, the rotation direction of the E vector of each circularly polarized wave is chosen to coincide with the rotation direction of the electron orbits in the magnetic field. In accordance with the dispersion relationship described above, the circularly polarized electromagnetic wave(s) of frequency xcfx89, are able to propagate from the chamber end to the zone where the magnetic field is approximately B0.
When a feed, which can be any mixture having both high mass and low mass constituents, is introduced into the chamber it will be subjected to ECH. Specifically, at the zone where the magnetic field is approximately B0 (i.e. the resonance zone), electrons in the zone are accelerated by the circularly polarized electromagnetic waves. The accelerated electrons then collide with neutrals, ions and other electrons from the feed and the collisions result in the ionization of neutrals and the heating of the electrons. The ionization and heating at the resonance zone initiates and maintains a multi-species plasma having ions of relatively high mass to charge ratio (M1) and ions of relatively low mass to charge ratio (M2) in the chamber.
The device further includes one or more electrodes for creating an electric field that is radially oriented within the chamber. Specifically, the electrode(s) establish a positive voltage (Vctr) along the longitudinal axis and a substantially zero potential at the wall of the chamber. With the crossed electric and magnetic fields, ions having a relatively low mass to charge ratio (M2) generated at the resonance zone are confined inside the chamber and transit through the chamber exiting at one of the chamber ends. On the other hand, ions generated at the resonance zone having a relatively high mass to charge ratio (M1) are not so confined. Instead, these larger mass ions strike a high mass ion collector mounted on the inside of the wall near the resonance zone before completing their transit through the chamber. Specifically, for a high mass ion collector that is at a distance xe2x80x9cacxe2x80x9d from the longitudinal axis, ions having a mass (M1) that is greater than a cut-off mass, Mc (M1 greater than Mc) will be collected at the wall near the resonance zone, where
Mc=eac2(Bc)2/8Vctr
wherein xe2x80x9cexe2x80x9d is the ion charge and Bc is the magnetic field at the ends of the high mass ion collector.
In another embodiment of the present invention, a radial electric field is generated in a chamber as described above. Also, coils are provided to generate an axially aligned, uniform magnetic field having magnetic field strength, B0, in the chamber. In this embodiment, a high frequency, polarized electromagnetic wave is launched into the chamber along a substantially radial path to initiate and maintain a plasma in the chamber via collisional absorption.
A pair of spaced apart reflectors are positioned to surround the plasma and establish a high Q cavity therebetween. With this cooperation of structure, the electromagnetic wave can be launched into the chamber for travel back and forth between the reflectors. Each time the wave travels between reflectors, it interacts with the plasma, heating the plasma via collisional absorption. Once generated, the plasma is separated in the crossed electric and magnetic fields as described above.